Preload users instead of querying separately when searching for comments (#13490)

* Preload users instead of querying separately

* Add conditional to comment.user

app/serializers/search/postgres_comment_serializer.rb

@@ -3,8 +3,8 @@ module Search
   class PostgresCommentSerializer < ApplicationSerializer
     attribute :id, &:search_id
 
-    attributes :path do |comment, params|
-      user = params[:users][comment.user_id]
+    attributes :path do |comment|
+      user = comment.user
 
       if user
         "/#{user.username}/comment/#{comment.id_code_generated}"
@@ -19,7 +19,7 @@ module Search
 
     attribute :class_name, -> { "Comment" }
 
-    attribute :highlight do |comment, _params|
+    attribute :highlight do |comment|
       {
         body_text: [comment.pg_search_highlight]
       }
@@ -40,9 +40,14 @@ module Search
 
     # NOTE: not using the `NestedUserSerializer` to avoid hitting Redis to
     # fetch the cached value
-    attribute :user do |comment, params|
-      user = params[:users][comment.user_id]
-      user.slice(:name, :profile_image_90, :username).symbolize_keys
+    attribute :user do |comment|
+      user = comment.user
+
+      if user
+        user.slice(:name, :profile_image_90, :username).symbolize_keys
+      else
+        {}
+      end
     end
   end
 end

app/services/search/postgres/comment.rb

@@ -15,14 +15,6 @@ module Search
       ].freeze
       private_constant :ATTRIBUTES
 
-      USER_ATTRIBUTES = %i[
-        id
-        name
-        profile_image
-        username
-      ].freeze
-      private_constant :USER_ATTRIBUTES
-
       DEFAULT_PER_PAGE = 60
       private_constant :DEFAULT_PER_PAGE
 
@@ -55,6 +47,7 @@ module Search
         per_page = [(per_page || DEFAULT_PER_PAGE).to_i, MAX_PER_PAGE].min
 
         relation = ::Comment
+          .includes(:user)
           .where(
             deleted: false,
             hidden_by_commentable_user: false,
@@ -69,33 +62,12 @@ module Search
 
         results = relation.page(page).per(per_page)
 
-        # NOTE: [@rhymes/atsmith813] an earlier version used `.includes(:user)`
-        # to preload users, unfortunately it's not possible in Rails to specify
-        # which fields of the included relation's table to select ahead of time.
-        # The `users` table is massive (115 columns on March 2021) and thus we
-        # shouldn't load it all in memory just to select a few fields.
-        # For these reasons I decided to avoid preloading altogether and issue
-        # an additional SQL query to load User objects
-        # (see https://github.com/forem/forem/pull/4744#discussion_r345698674
-        # and https://github.com/rails/rails/issues/15185#issuecomment-351868335
-        # for additional context)
-        user_ids = results.pluck("comments.user_id")
-        users = find_users(user_ids)
-
-        serialize(results, users)
+        serialize(results)
       end
 
-      def self.find_users(user_ids)
-        ::User
-          .where(id: user_ids)
-          .select(*USER_ATTRIBUTES)
-          .index_by(&:id)
-      end
-      private_class_method :find_users
-
-      def self.serialize(results, users)
+      def self.serialize(results)
         Search::PostgresCommentSerializer
-          .new(results, params: { users: users }, is_collection: true)
+          .new(results, is_collection: true)
           .serializable_hash[:data]
           .pluck(:attributes)
       end